I know this is an old post from 2009, but I just took my 250GB G-Drive Q that I purchased in 2007, and successfully swapped out the original Hitachi Deskstar 250GB HD with a 1TB Samsung Spinpoint F3 drive. I connected my iMac using firewire 800, formatted it to Mac OS, and it's running perfectly... actually quieter and smoother than before, and feels cooler to the touch.
Note that G-Tech support says that you can upgrade the G-Drive Q up to 1TB. See copy/paste from an email G-Tech support sent me:
Dear Kevin,
Thank you for contacting the Hitachi GST Technical Support Center.
The Q line of drives should support up to 1TB drives. That was the highest we went before moving onto our Gen 4 units.
You shouldn't have any problems upgrading as long as the interface inside is a SATA drive.
Regards,
Hitachi GST Technical Support Center